We propose to examine behavioral interactions at the level of single, identified neurons in the nervous system of the medicinal leech. We will first show how various behaviors interact by determining the probability of eliciting each of four different behaviors--swimming, walking, shortening and local bending--in response to mechanosensory stimuli. The variability in the responses will provide us with a measure of behavioral set of the animal. We will then show how the four behaviors interact by delivering pairs of stimuli, either simultaneously or in sequence, which individually would elicit one of the behaviors very reliably. If the pairs of behaviors are mutually exclusive, these studies will measure the animal's behavioral choice; from different pairings, we will determine the animal's behavioral hierarchy. The behaviors may prove to be compatible, so that elements are noninterfering or coordinated. It appears that swimming and crawling are coordinated, that local bending does not interfere with crawling or swimming, and that all other behaviors are organized into a hierarchy. We will also determine to what extent these behavioral interactions are modified by neuromodulators, particularly serotonin, and by learning. In parallel studies, we will characterize the cellular and network properties responsible for initiating and generating each of the four behaviors. This process is nearly complete for two of the behaviors, swimming and local bending, and is well begun for the other two. We will impale and record from two or three neurons simultaneously in semi-intact, behaving leeches or in isolated nerve cords. We will test the neuronal characterizations for the completeness by comparing the activity pattern recorded in the animal to a digitized model network, under both normal and perturbed conditions. The perturbations will be: 1) polarizing one to three neurons at a time, or 2) eliminating the interactions between neurons altogether by photoablation. We will then determine the interactions between neurons in the four behavioral circuits, to find the neuronal basis for the compatibility, coordination, or mutual exclusivity of the behaviors. Based on this information, we will be able to determine the site and nature of neuronal changes responsible for the modification of these interactions by neuromodulators and by learning. Such neuronal mechanisms are likely to be found in all animals, including humans, when behaviors interact.